It turns out there are hidden advantages to having the core APIs the same between Windows 10 and Windows 10 Mobile.
Developer Steven Countryman is creating a universal app which will automatically change the desktop wallpaper on Windows 10 and it turns out the same API will also change the start screen wallpaper on Windows 10 Mobile, enabling a feature I think Windows Phone users have been longing after for a long time.
At present the beta only changes the wallpaper to the daily Bing background, but Steven promises the eventual app will do a lot more.
To join the beta express your interest in this Reddit thread here.